What masters cylinder to use for 73 dart disc brakes and 8.8 ford rear end disc brakes

15/16 for manual brakes, and you will need a proportioning valve. Your dash should have a "brake-warning lamp." if it does, then use the Combination valve from any disc-brake A-body.
If you have 9" drums on the rear, I humbly recommend an upgrade to 10 inchers with the appropriate proportioning..